1. Field of the Invention;
The present invention relates to a control device for an electric vehicle. More specifically, the present invention relates to a control device for controlling a rotational speed of a DC shunt motor or a compound motor comprising a shunt field coil used to power an electric car.
2. Descrption of the Prior Art;
Either of a DC series motor and a DC shunt motor may be used to power an electric car. As to well known, the rotational speed of a DC series motor is proportional to a terminal voltage of the motor. Therefore, by way of a speed control device of an electric car using a DC series motor, a resistor switching control circuit or a thyristor chopper circuit is connected in series therewith, to control the motor terminal voltage. The series resistance value is controlled by the resistor switching control circuit or a conduction angle of an on/off control by the thyristor chopper is controlled by a speed control apparatus, such as an accelerator.
As far as the electric characteristic is concerned, a DC series motor is most suited for speed control. Nevertheless, a great problem encountered in an electric car using a series motor is that efficiency of the series motor is low as compared with a shunt motor. Generally speaking, efficiency of the shunt motor is about 10 to 15 per cent higher than that of the series motor. Thus, it is desired that a electric vehicle such as an electric car using a storage battery power source be driven by a shunt motor, rather than by a series motor.
Another problem encountered in an electric car using a DC series motor is that the abovementioned resistor switching control circuit is constructed in as many switching control stages as possible in order to make smooth variation of the speed, thereby resulting in a complicated structure. An electric car using a series motor is further disadvantageous in that when the electric car is controlled in a relatively low speed a resistor of a given value is connected in series with the motor, which, undesirably dissipates electric power and decreases the efficiency.
On the other hand, in an electric car using a series motor controlled by means of a thyristor chopper, the motor is energized by an on/off current or a ripple current. Therefore, the commutating characteristic of the motor becomes poor and is liable to cause a power loss. If a commutation reactor is used in order to avoid the abovementioned disadvantage, a power loss also results due to other causes such as iron loss or copper loss by the reactor. Another problem encountered in connection with an electric car using a series motor energized via a thyristor chopper is a turnoff failure. To provide for securing a turn-off operation of the chopper entails a problem of complicated circuitry and thus of a higher cost. Another great problem encountered in connection with an electric car using a series motor energized via a thyristor is that components used in the chopper make undesired noise, which is often large and causing discomfort to a driver of the car. More specifically, when a thyristor chopper is operated to turn on and off the direct current by means of a thyristor as a switching device, various components vibrate due to electrical oscillation generated in the chopper, and make a noise, often large enough to make driver feel uncomfortable.
Most disadvantages described hereinabove will be eliminated by employment of a shunt motor in an electric car. The largest advantage in employing a shunt motor in an electric car is that power consumption of a shunt motor is much less as compared to a series motor, such an advantage is very desirable in an electric car using a storage battery as a power source. Nevertheless, as is well known, speed control of a shunt motor is relatively difficult. One approach for speed control of a shunt motor is to change a current flowing through a shunt field coil by means of a speed control device. As is well known, with a shunt motor the smaller the field current is, the higher the rotational speed of the armature becomes. However, this approach merely provides a very narrow speed controllable range, and therefore cannot cover so wide a controllable range as to meet the requirement of an electric car.
Another approach for speed control of a shunt motor is to control the terminal voltage at the armature. Just as in case of a series motor, a rotational speed of the shunt motor is proportional to the armature terminal voltage, although in the shunt motor the maximum speed thereof is determined by the field current. Therefore, the approach for control of the terminal voltage described in connection with speed control of a series motor may be employed for the purpose of speed control of a shunt motor. However, even in the case of controlling the armature terminal voltage of a shunt motor, the same problems as encountered in control of the terminal voltage of the series motor will be again encountered. Thus, an improved speed control device is desired for use in a DC motor comprising a shunt field coil, such as a shunt motor, which is of high efficiency, of simplicity in structure, and creates little noise.
Typically, a resistor switching control circuit is also used for the purpose of controlling the armature terminal voltage of the shunt motor. The resistor switching control circuit comprises one or more series resistor connected between a power supply and a shunt motor. The resistors are shunted or short circuited by a relay operable in a ganged fashion with an accelerator pedal so that depression of the accelerator pedal to a certain degree causes the relay to be activated and thereby shunting the series resistor to increase the armature terminal voltage and thus to increase the motor speed to a value as determined by the current fed to the field coil thereof. Therefore, assuming that a driver of an electric car employing such a resistor-switched shunt motor depresses abruptly an accelerator pedal to a maximum degree from an initial position at the time of starting the car, an armature is supplied with a power through a minimum value of series resistance and thus an armature current tends to reach immediately a value for a maximum speed of the motor. As a result steering stability of the car decreases and both the motor and a control system therefor are adversely affected. Thus, an improved speed control device is desired for use in a DC motor having a shunt field coil, such as a shunt motor, which allows for stable of steering of the car.
If an electric car employs a shunt motor, which is selectively adapted to drive the car upon energization thereof by way of an ordinary driving mode it will generate a power to the power supply by way of a regenerative braking mode. When the car is running and the accelerator pedal is released armature is rotated due to inertia of the running car and a regenerative power is generated to charge a storage battery of the power supply. Assuming that the field coil current is constant or is not varied, the higher the rotational speed of the armature the higher the generated voltage and thus the greater the regeneration to the battery. Therefore, if the accelerator pedal is released abruptly while the car is running in a high speed, the regenerative braking is abruptly effective and a very dangerous situation results. At the same time a large current flows through the armature and adversely affects the motor.
Therefore, it is a primary object of the present invention to provide a speed control device for an electric vehicle using a DC motor, comprising a shunt field coil, such as a shunt motor or a compound motor, such DC motors are characterized by high efficiency, have a simple structure, are less expensive, and generate little noise.
It is another object of the present invention to provide a speed control device for an electric vehicle using a DC motor comprising a shunt field coil, which stabilizes the steering of the vehicle.
It is a further object of the present invention to provide a speed control device for an electric vehicle using a DC motor comprising shunt field coil, in which a proper regenerative braking is obtained irrespective of a rotational speed or an armature current.
Other objects, features and advantages of the present invention will be better understood from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.